In order to prevent a typical cylindrical lithium ion secondary battery from exploding in the case of overcharging, it is provided with a safety vent, which deforms when the internal pressure rises due to overcharging, and a circuit board, which interrupts the current as the safety vent deforms. The safety vent and the circuit board are also referred to as CIDs (current interruption devices) as a whole and constitute the cap assembly.
The operation of the safety vent and the circuit board of a cylindrical lithium ion secondary battery will now be described in more detail.
When a cylindrical lithium ion secondary battery is overcharged, the electrolyte evaporates approximately from the upper region of the electrode assembly and the resistance begins to increase. In addition, lithium precipitates and deformation begins to occur approximately from the central region of the electrode assembly. The increase of resistance in the upper region of the electrode assembly causes local heating and abruptly raises the battery temperature.
In this state, the action of cyclo hexyl benzene (CHB) and biphenyl (BP) (electrolyte additive), which generally decomposes and generate gas in the case of overcharging, rapidly increases the internal pressure. Such internal pressure pushes the safety vent, which is one of the components of the cap assembly, outwards (i.e., deforms it outwards). As a result, the circuit board positioned thereon is fractured and interrupts the current. Particularly, the wiring pattern formed on the circuit board is broken and no current flows any longer. Such interruption of current ends the overcharging state and prevents the battery from heating, leaking, smoking, exploding, or catching fire.
When the internal pressure of the battery rises above a critical level due to overcharging, the safety vent itself is tom off and evacuates internal gas to the exterior.
Meanwhile, a void volume or dead volume generally exists inside the battery. In particular, the empty space between the electrode assembly and the cap assembly or that inside the center pin may be referred to as a void volume. Such a void volume is thought to be one of the reasons the time of deformation or fracture of the safety vent is delayed. In other words, the void volume is thought to delay the current interruption time and degrade the stability of the battery.
It is known in the art that, when the safety vent inside the battery deforms (or the circuit board fractures) at a pressure of about 5-11 kgf/cm2 and the void volume is about 2 ml, for example, the amount of gas necessary for deformation of the safety vent is about 10-22 ml, although there may be some variance depending on the type of the battery. However, even when cyclo hexyl benzene (CHB) completely decomposes, which is included in the electrolyte at a ratio of 0.7% based on calculation, gas of about 4.116 ml is generated and, even when 0.3% of biphenyl (BP) completely decomposes, gas of about 1.833 ml is generated. In addition, about 1.5 ml of gas is additionally generated in the degassing process. The total sum of gas from three different sources, however, is no more than about 7.449 ml and applies a force of about 3.75 kgf/cm2 to the safety vent. In summary, although a pressure of about 5-11 kgf/cm2 is necessary to deform the safety vent or break the circuit board in the case of overcharging, the void volume can actually provide only about 3.75 kgf/cm2. As a result, the safety vent is not operated or the operation time is delayed. This means that the current interruption time is delayed in the case of overcharging. The resulting problem is that overcharging further proceeds as long as time is delayed, the battery temperature further rises, and the battery is very likely to explode or catch fire. Although the amount of gas generated in the case of overcharging may become larger by increasing the amount of cyclo hexyl benzene (CHB) or biphenyl (BP), which is an additive to the electrolyte, there is a trade-off between degradation of capacity, life, and quality of the battery.